1. Field of the Invention
This invention relates generally to subterranean formation treatments and, more specifically, to hydraulic fracturing treatments for subterranean formations. In particular, this invention relates to use of relatively lightweight and/or substantially neutrally buoyant particles as proppant material in hydraulic fracturing treatments.
2. Description of the Related Art
Hydraulic fracturing is a common stimulation technique used to enhance production of fluids from subterranean formations. In a typical hydraulic fracturing treatment, fracturing treatment fluid containing a solid proppant material is injected into the formation at a pressure sufficiently high enough to cause the formation or enlargement of fractures in the reservoir. During a typical fracturing treatment, proppant material is deposited in a fracture, where it remains after the treatment is completed. After deposition, the proppant material serves to hold the fracture open, thereby enhancing the ability of fluids to migrate from the formation to the well bore through the fracture. Because fractured well productivity depends on the ability of a fracture to conduct fluids from a formation to a wellbore, fracture conductivity is an important parameter in determining the degree of success of a hydraulic fracturing treatment.
Hydraulic fracturing treatments commonly employ proppant materials that are placed downhole with a gelled carrier fluid (e.g., aqueous-based fluid such as gelled brine). Gelling agents for proppant carrier fluids may provide a source of proppant pack and/or formation damage, and settling of proppant may interfere with proper placement downhole. Formulation of gelled carrier fluids usually requires equipment and mixing steps designed for this purpose.
In the disclosed method, the application of relatively lightweight and/or substantially neutrally buoyant particulate material as a fracture proppant particulate advantageously may provide for substantially improved overall system performance in hydraulic fracturing applications. By xe2x80x9crelatively lightweightxe2x80x9d it is meant that a particulate has a density that is substantially less than a conventional proppant particulate material employed in hydraulic fracturing operations, e.g., sand or having a density similar to these materials. By xe2x80x9csubstantially neutrally buoyantxe2x80x9d, it is meant that a particulate has a density sufficiently close to the density of a selected ungelled or weakly gelled carrier fluid (e.g., ungelled or weakly gelled completion brine, other aqueous-based fluid, or other suitable fluid) to allow pumping and satisfactory placement of the proppant particulate using the selected ungelled or weakly gelled carrier fluid. For example, urethane resin-coated ground walnut hulls having a specific gravity of from about 1.25 to about 1.35 grams/cubic centimeter may be employed as a substantially neutrally buoyant proppant particulate in completion brine having a density of about 1.2. It will be understood that these values are exemplary only. As used herein, a xe2x80x9cweakly gelledxe2x80x9d carrier fluid is a carrier fluid having minimum sufficient polymer, viscosifier or friction reducer to achieve friction reduction when pumped down hole (e.g., when pumped down tubing, work string, casing, coiled tubing, drill pipe, etc.), and/or may be characterized as having a polymer or viscosifier concentration of from greater than about 0 pounds of polymer per thousand gallons of base fluid to about 10 pounds of polymer per thousand gallons of base fluid, and/or as having a viscosity of from about 1 to about 10 centipoises. An ungelled carrier fluid may be characterized as containing about 0 pounds per thousand gallons of polymer per thousand gallons of base fluid.
Advantageously, in one embodiment use of substantially neutral buoyant particulate material may eliminate the need for gellation of carrier fluid, thus eliminating a source of potential proppant pack and/or formation damage. Furthermore, a relatively lightweight particulate material may be easier to place within a targeted zone due to lessened settling constraints. Elimination of the need to formulate a complex suspension gel may mean a reduction in tubing friction pressures, particularly in coiled tubing and in the amount of onlocation mixing equipment and/or mixing time requirements, as well as reduced costs. Furthermore, when treated to have sufficient strength (e.g., by substantially filling the permeable porosity of a porous particle with resin or hardener), the disclosed relatively lightweight proppant particles may be employed to simplify hydraulic fracturing treatments performed through coil tubing, by greatly reducing fluid suspension property requirements. Downhole, a much reduced propensity to settle (as compared to proppant particulates) may be achieved. In this regard, the disclosed substantially neutral buoyancy proppant material may be advantageously employed in any deviated well having an angle of deviation of between about 0 degree and about 90 degrees with respect to the vertical. However, in one embodiment, the disclosed particulate material may be advantageously employed in horizontal wells, or in deviated wells having an angle with respect to the vertical of between about 30 degrees and about 90 degrees, alternatively between about 75 degrees and about 90 degrees. Thus, use of the disclosed relatively lightweight and/or substantially neutrally buoyant particulate materials disclosed herein may be employed to achieve surprising and unexpected improvements in fracturing methodology, including reduction in proppant pack and/or formation damage, and enhancement of well productivity.
In another embodiment, protective and/or hardening coatings, such as resins described elsewhere herein may be selected to modify or customize the specific gravity of a selected base particulate proppant material, e.g., ground walnut hulls, etc. Modification of particulate specific gravity (i.e., to have a greater or lesser specific gravity) may be advantageously employed, for example, to provide proppant particulates of customized specific gravity for use as a substantially neutrally buoyant particulate with a variety of different weight or specific gravity carrier fluids. In yet another embodiment, protective and/or hardening-type coatings may be optionally curable to facilitate proppant pack consolidation after placement. In this regard, curable resins are know to those of skill in the art, and with benefit of this disclosure may be selected to fit particular applications accordingly.
The disclosed relatively lightweight and/or substantially neutrally buoyant particulate proppant materials may be employed with carrier fluids that are gelled, non-gelled, or that have a reduced or lighter gelling requirement as compared to carrier fluids employed with conventional fracture treatment methods. In one embodiment employing one or more of the disclosed substantially neutrally buoyant particulate materials and a brine carrier fluid, mixing equipment need only include such equipment that is capable of (a) mixing the brine (dissolving soluble salts), and (b) homogeneously dispersing in the substantially neutrally buoyant particulate material.
In one embodiment, a substantially neutrally buoyant particulate proppant material may be advantageously pre-suspended and stored in a storage fluid (e.g. brine of near or substantially equal density), and then pumped or placed downhole as is, or diluted on the fly.
In one respect, disclosed is a hydraulic fracturing method for a well penetrating a subterranean formation, including introducing a relatively lightweight and/or substantially neutral density particulate proppant material into the well. Individual particles of the particulate material optionally may have a shape with a maximum length-based aspect ratio of equal to or less than about 5. Individual particles may also be optionally coated with protective materials such as resins and/or hardeners, for example, xe2x80x9c2ACxe2x80x9d phenol formaldehyde hardener from BORDEN CHEMICAL. Examples of suitable relatively lightweight and/or substantially neutrally buoyant materials for use in aqueous based carrier fluids include, but are not limited to, ground or crushed nut shells, ground or crushed seed shells, ground or crushed fruit pits, processed wood, or a mixture thereof. Optional protective coatings for coating at least a portion of individual particles of such relatively lightweight and/or substantially neutrally buoyant materials include, but are not limited to at least one of phenol formaldehyde resin, melamine formaldehyde resin, urethane resin, or a mixture thereof. Other optional coating compositions known in the art to be useful as hardeners for such materials (e.g., coating materials that function or serve to increase the elastic modulus of the material) may be also employed in conjunction or as an alternative to protective coatings, and may be placed underneath or on top of one or more protective coatings. It will be understood by those of skill in the art that such protective and/or hardening coatings may be used in any combination suitable for imparting desired characteristics to a relatively lightweight and/or substantially neutrally buoyant particulate proppant material, including in two or more multiple layers. In this regard successive layers of protective coatings, successive layers of hardening coatings, alternating layers of hardening and protective coatings, etc. are possible. Mixtures of protective and hardening coating materials may also be possible.
In another respect, disclosed is a relatively lightweight and/or substantially neutrally buoyant fracture proppant material for use in a hydraulic fracturing treatment that is a ground or crushed walnut shell material that is coated with a resin to substantially protect and water proof the shell. Such a material may have a specific gravity of from about 1.25 to about 1.35, and a bulk density of about 0.67. In one exemplary case, size of such a material may be about 12/20 US mesh size. In another exemplary case, sizes may range from about 4 mesh to about 100 mesh. Advantageously, in some embodiments, such ground walnut shells may serve to attract fines and formation particles by their resinous nature. In one embodiment for the manufacture of such particles for proppant applications, an optional hardener may be applied to a ground walnut shell material first followed by a urethane coating as described elsewhere herein that may vary in amount as desired. For example, such a coating material may be present in an amount of from about 1% to about 20%, alternatively from about 10% to about 20% by weight of total weight of individual particles. Alternatively, such a coating material may be present in an amount of from about 2% to about 12% by weight of total weight of individual particles. Amount of resin may depend, for example, on price and application. In this regard, particulates may be first sprayed or otherwise coated with a hardener, and a coating may be applied to be about 12% by weight of total weight of the particle.
In one embodiment, the disclosed relatively lightweight particulate proppant material may be introduced or pumped into a well as neutrally buoyant particles in, for example, a saturated sodium chloride solution carrier fluid or a carrier fluid that is any other completion or workover brine known in the art, for example, having a specific gravity of from about 1 to about 1.5, alternatively from about 1.2 to about 1.5, further alternatively about 1.2, thus eliminating the need for damaging polymer or fluid loss material. In one embodiment, such a material may be employed as proppant material at temperatures up to about 150xc2x0 F., and pressures up to about 1500 psi. However, these ranges of temperature and closure stress are exemplary only, it being understood that the disclosed materials may be employed as proppant materials at temperatures greater than about 150xc2x0 F. and/or at closure stresses greater than about 1500 psi, it also being understood with benefit of this disclosure that core and/or layer material/s may be selected by those of skill in the art to meet and withstand anticipated downhole conditions of a given application.
Advantageously, in one embodiment the low specific gravity of the relatively lightweight proppant material may be taken advantage of to result in a larger width for the same loading (i.e., pound per square foot of proppant) to give much larger total volume and increased width for the same mass. Alternatively, this characteristic allows for smaller volumes of proppant material to be pumped while still achieving an equivalent width.
In yet another respect, disclosed is a method of fracturing a subterranean formation, including: injecting a particulate material into the subterranean formation at a pressure above the fracturing pressure of the formation; wherein at least a portion of the individual particles of the particulate material each include a first material selected from at least one of ground or crushed nut shells, ground or crushed seed shells, ground or crushed fruit pits, processed wood, or a mixture thereof; and wherein at least a portion of the individual particles of the particulate material each includes a core component of the first material at least partially surrounded by at least one layer component of second material, the second material including a protective or hardening coating.
In yet another respect, disclosed is a method of fracturing a subterranean formation, including: introducing a particulate material suspended in a carrier fluid into the subterranean formation at a pressure above a fracturing pressure of the subterranean formation. In this method, at least a portion of the individual particles of the particulate material may be substantially neutrally buoyant in the carrier fluid and may include: a core component of a first material selected from at least one of ground or crushed nut shells, ground or crushed seed shells, ground or crushed fruit pits, processed wood, or a mixture thereof; and at least one layer component of second material surrounding the core component, the second material including a protective or hardening coating.